Alternate fuels have been developed to mitigate the rising prices of conventional fuels, to reduce dependence on imported fuels, and for reducing production of pollutants, such as CO2. For example, alcohol and alcohol-based fuel blends have been recognized as attractive alternative fuels, in particular for automotive applications. However, alcohol and alcohol-based fuels are less volatile than diesel, and as such may not evaporate effectively during engine cranking at cold-start conditions. Incomplete vaporization of the alcohol and alcohol-based fuels may reduce fuel economy and degrade emissions.
Various approaches are provided for increasing fuel vaporization during cold-start. In one example, as shown by Samejima in JP 2009002314A, an engine cranking speed is adjusted. In particular, a higher engine cranking speed is applied when the alcohol concentration of the injected fuel is high and the ambient temperature is low. In another example approach shown by Kuroki in JP 2008232007, a starter motor speed is increased when there is an issue with fuel vaporization. In a further example, Ulrey et al. in U.S. Pat. No. 9,346,451 disclose a method of cranking the engine unfueled at a lower than normal speed such that the heat generated in the compression stroke of a cylinder may be transferred to cylinder walls, thereby expediting engine warm-up.
However, the inventors herein have recognized potential issues with such approaches. In the approaches shown by Samejima and Kuroki, the starter speed is increased to rapidly reduce the intake manifold pressure since the lower pressure assists in fuel vaporization. However, the rapid reduction in manifold pressure via the increasing of the starter speed also reduces the time available for vaporizing the fuel. Consequently, it may be difficult to optimize the starter speed for both the manifold pressure and the fuel alcohol content. Also, increased cranking speed during engine start may result in engine flares. An optimal amount of vaporized fuel may be desired to maintain combustion stability. Incomplete fuel vaporization may further lead to cylinder misfiring events. In the approach shown by Ulrey et al., since the engine is cranked unfueled at a lower cranking speed, once fueling is initiated, fuel blends with a higher alcohol content may not get sufficient time for vaporization before combustion is initiated.
In one example, the issues described above may be addressed by an engine method comprising: for a number of engine cycles since a first engine cycle of an engine cold-start, cranking the engine via a starter motor with cranking speed decreased relative to a nominal cranking speed while injecting fuel and disabling spark; and after the number of engine cycles with injected fuel compressed and expanded, increasing the cranking speed to the nominal cranking speed and initiating spark. In this way, by cranking the engine via a starter motor at a lower cranking speed while fueling the engine without spark until a number of engine cycles are completed, sufficient time may be provided to vaporize the fuel and provide a homogeneous air-fuel mixture. By raising the cranking speed and resuming spark after the number of engine cycles have elapsed, the intake manifold pressure may be lowered and cylinder combustion can be performed with a higher degree of fuel vaporized before combustion is initiated.
As one example, during cold-start conditions, a starter motor may be actuated to crank the engine. The cranking speed may be lowered relative to a nominal cranking speed while fuel is injected into the engine with spark disabled for a number of engine cycles. The number of engine cycles and the cranking speed may be selected based on the alcohol content of the injected fuel and the ambient temperature so as to enable a larger portion of the fuel to be vaporized by the time spark is enabled. As an example, the cranking speed may be lowered to 150 rpm, and the engine may be fueled with no spark for two complete engine cycles (e.g., the first two engine cycles since engine start is initiated). On the subsequent engine cycle (e.g., the third engine cycle since the engine is started), the cranking speed may be raised, for example to 250 rpm, and spark may be resumed. To further improve fuel vaporization, fuel injection timing may be adjusted to extend up till the spark event. For example, an end of fuel injection timing may be shifted from bottom dead center (BDC) of the intake stroke to top dead center (TDC) of compression stroke.
In this way, by lowering cranking speed to below the nominal speed, a larger time window is provided for fuel vaporization. Also, by using a lower engine cranking speed, engine speed flares may be reduced. By continuing fuel injection until spark, a higher amount of fuel may be injected which may result in an increase in the amount of vaporized fuel available for combustion. The technical effect of deactivating spark until a defined number of fueled engine cranking cycles have elapsed is that each cylinder may be conditioned with vaporized fuel and upon activating spark after accumulation of an optimal amount of pre-vaporized fuel, combustion stability may be improved. By improving combustion stability, misfire event occurrence and unburned hydrocarbon emissions during engine starts may be reduced. By increasing the cranking speed after the number of engine cycles have elapsed, the desired intake manifold pressure may be attained, facilitating combustion. By adjusting the cranking speed, fuel injection profile, and the number of non-firing cycles based on the alcohol content of the fuel, vaporization of any variety of alcohol fuel may be optimized. Overall, by increasing the degree of fuel alcohol vaporization, engine performance, fuel economy, and emissions quality may be increased.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.